Mechanism of Burn-off of Tungsten Carbide Particles During Weld Overlay

Literature Overview

This paper by Qu Shi-Yao, Wang Xin-Hong, Zou Zeng-Da, and Liu Xue-Mei from the School of Materials Science and Engineering at Shandong University, published in the Welding Journal (焊接学报) in 2001, addresses a critical and practically challenging problem in the field of hardfacing and wear-resistant overlay welding. The burn-off, or loss, of tungsten carbide (WC) particles during the welding process is one of the most persistent issues that engineers face when applying WC-based hardfacing coatings to tooling and machinery components. The study systematically investigates the mechanisms governing WC particle degradation under the extreme thermal conditions of arc welding.

Core Technical Points

The fundamental challenge in WC-based hardfacing lies in the thermodynamic instability of WC under the high-temperature, high-energy-density conditions of welding. WC decomposes at temperatures above approximately 1,000 °C, and the arc temperature in most overlay welding processes far exceeds this threshold. The paper identifies several key mechanisms contributing to WC burn-off:

• Thermal decomposition: WC decomposes into W₂C and free carbon at elevated temperatures, with further decomposition of W₂C into elemental tungsten and graphite at still higher temperatures.
• Oxidation and volatilization: Tungsten oxide species (WO₃) form readily in the presence of oxygen and can volatilize at relatively low temperatures (above 800 °C), leading to permanent loss of tungsten from the weld pool.
• Dilution by base metal: The molten base metal dilutes the weld pool, reducing the effective concentration of WC and potentially causing partial dissolution of the carbide particles.
• Mechanical spattering: High-energy particles can be ejected from the weld pool due to arc pressure and plasma turbulence.

The paper provides a quantitative framework for understanding how process parameters influence the degree of burn-off, which is essential for process optimization.

Interpretation of Technical Points

The thermodynamic decomposition pathway of WC follows a well-established sequence:

1. WC → W₂C + C (onset around 1,000 °C)
2. W₂C → W + C (above approximately 1,200 °C)
3. W + O₂ → WO₃ (volatile above 800 °C)
4. WO₃ + H₂O → H₂WO₄ (volatile acid formation)

The practical implication is that any welding process capable of maintaining the weld pool temperature below 1,000 °C would theoretically preserve WC integrity, but this is not achievable with conventional arc processes. The study highlights that the burn-off ratio is not simply a function of peak temperature but is governed by the time-temperature history experienced by each particle as it travels through the weld pool.

A key insight from the research is that the particle size distribution significantly affects burn-off behavior. Smaller WC particles (below 10 μm) tend to dissolve more readily into the weld pool due to their higher surface-to-volume ratio, while larger particles (above 50 μm) are more susceptible to mechanical spattering. There exists an optimal particle size range (approximately 20–40 μm) where the balance between dissolution and mechanical retention is most favorable.

Process and Standards Analysis

The paper's findings have direct implications for the selection of overlay welding processes. Processes with lower heat input and shorter residence time in the weld pool are inherently more favorable for WC retention. This explains why:

• Plasma transferred arc (PTA) welding is preferred over submerged arc welding (SAW) for WC-based coatings, as PTA provides a more focused heat source and allows for controlled powder feeding rates.
• Laser cladding achieves the lowest burn-off ratios due to the extremely short interaction time between the laser beam and the workpiece.
• Flux-cored arc welding (FCAW) with WC-containing flux can achieve moderate retention through the self-shielding flux layer, which limits oxidation.

The study also discusses the role of shielding atmosphere in mitigating oxidation. Inert gas shielding (argon or argon-helium mixtures) is essential, and the addition of small amounts of hydrogen (1–3%) can reduce the vapor pressure of tungsten oxides by maintaining a reducing atmosphere at the weld pool surface.

Integration with Engineering Practice

In practical hardfacing applications, the burn-off problem manifests as reduced hardness and accelerated wear of the overlay layer. For example, in the hardfacing of digester tools in mining operations, WC-based coatings are expected to deliver hardness values above 1,500 HV. However, without proper process control, the actual hardness may drop to 1,000–1,200 HV due to significant WC loss.

The study's recommendations for minimizing burn-off include:

• Using low heat input settings (below 0.8 kJ/mm for SAW, below 15 A/mm for PTA).
• Applying multiple thin layers (each below 1.5 mm) rather than a single thick deposit.
• Using preheated backing plates to reduce thermal gradient and minimize dilution.
• Employing a powder feed rate that maintains a consistent powder-to-gas ratio in PTA applications.

From a quality control perspective, the residual WC content in the overlay can be assessed through metallographic examination, X-ray diffraction analysis, and hardness mapping. A minimum WC retention of 40–50% is generally considered acceptable for industrial applications, though this varies with the specific service conditions.

Key Questions and Reflections

The paper raises several questions that remain relevant in contemporary practice. First, the extent to which the decomposition products (W₂C and free carbon) contribute to wear resistance is not fully resolved. In some applications, the presence of W₂C and carbide networks may still provide adequate hardness even when the original WC has partially decomposed. Second, the interaction between burn-off and cracking susceptibility is complex—reduced WC content may lower the carbon activity in the weld pool, potentially reducing the risk of microcracking, but this benefit must be weighed against the loss of wear resistance.

The study also implicitly addresses the economic dimension of WC burn-off. Tungsten is a rare and expensive element, and excessive burn-off represents a significant material cost. The paper's framework for optimizing process parameters to maximize WC retention has direct implications for cost-effective hardfacing operations.

Study Insights and Implications

This 2001 publication remains highly relevant because the fundamental thermodynamics of WC decomposition have not changed, even as welding technology has advanced. The mechanisms identified in this study—thermal decomposition, oxidation, dilution, and spattering—are still the primary concerns for engineers designing WC-based hardfacing processes today. The paper's systematic approach to quantifying burn-off provides a valuable analytical tool for process development and qualification.

For engineers working with bimetal products and hardfacing overlays, the key takeaway is that WC retention must be treated as a process-controlled variable rather than a material constant. Process selection, parameter optimization, and shielding strategy must all be considered in concert to achieve the desired level of carbide retention in the final overlay.

The study also highlights an important principle in overlay welding: the microstructure of the deposit is not simply a function of the filler material composition but is profoundly shaped by the thermal history imposed by the welding process. This principle extends to all alloy systems and is a cornerstone of weld overlay engineering.

In conclusion, the work by Qu Shi-Yao and colleagues provides a rigorous mechanistic understanding of WC burn-off that continues to inform process development and quality control in hardfacing applications. Engineers should treat the burn-off problem as a multi-factorial challenge requiring integrated control of heat input, shielding, particle size, and layer geometry to achieve the target microstructure and performance in WC-based overlay welds.